Motor drives and other switching power supplies are used to provide the multiphase AC output power to drive a load, such as an induction motor. Most conventional AC drives include a switching inverter output stage which converts power from a DC bus to provide AC output voltages to drive the load using an array of switches individually connected between a corresponding terminal of the DC bus and one of the output terminals. Pulse width modulation type switching control is employed to selectively actuate the inverter switches in order to create variable frequency, variable amplitude output voltage signals at the individual load terminals. Switching operation is typically controlled in closed-loop fashion according to one or more setpoint signals or values and also according to feedback from the inverter output. In a typical case, a motor drive receives a speed or position setpoint, and may also receive a torque setpoint, together with feedback signals indicating the measured output voltage of the inverter and other operating conditions of the drive. One or more internal control loops are used to regulate output load motor speed, torque, voltage and/or current based on the received setpoints and feedback values.
Due to the pulse width modulated operation of the output inverter, however, accurate assessment of the voltage waveforms at the inverter output is difficult. Specifically, operation of the inverter switches at relatively high pulse width modulation frequencies, such as several kilohertz, causes the output voltage waveforms to include high frequency components. In addition, switching power supplies are noisy electrical environments, and the voltage output waveforms are thus not pure sinusoids. In many applications, moreover, the output rating of the driven load is relatively high, whereby the DC bus voltage and the AC output voltages are of relatively high values. Obtaining accurate feedback in these situations is therefore difficult, since the sensed signals are sometimes of very high voltage amplitudes, and the switching of large currents within the inverter creates a noisy environment in which it is difficult to accurately sense the output voltage signals. At the same time, however, high bandwidth sensing circuitry is required to be able to accurately reproduce the PWM voltage output waveforms for feedback purposes, and high-bandwidth sensing circuitry is more susceptible to noise. At very low modulation index operating levels, moreover, voltage waveform pulse widths are very narrow, and reflected wave effects as well as deadtime compensation effects become more significant, leading to increased difficulty in determination of the feedback signals. Accordingly, a need remains for improved methods and apparatus for sensing output voltages for control of switching inverters in motor drives and other power conversion applications.